Build: More GNUInstallDirs improvements These improvements enable build systems to use GNUInstallDirs to define custom directory variables. - The set_dir() macro was renamed to GNUInstallDirs_set_install_dir(), in keeping with the module's established macro naming convention. - Rather than detecting whether the prefix has changed, the new GNUInstallDirs_set_install_dir() macro instead examines whether the default for the variable in question has changed. This allows for more flexibility, since build systems may decide to change the defaults based on factors other than the prefix. It also enables the macro to work properly outside of the module. - The module now performs directory variable substitution within the body of GNUInstallDirs_get_absolute_install_dir(). - The JAVADIR variable is no longer included in GNUInstallDirs. That directory is not part of the GNU spec, and it turns out that various operating systems use different conventions for the location of Java classes. Instead, the variable is now implemented in our build system as a demonstration of the aforementioned GNUInstallDirs enhancements.